The phrase "a knapsack with" is correct and usable in written English.
It can be used when describing an object that is accompanied by or contains something else, typically in a context related to travel or carrying items.
Example: "She packed a knapsack with all her essentials for the hiking trip."
Alternatives: "a backpack containing" or "a bag filled with".
On his way back to Missouri, he fills a knapsack with cotton candy for his daughters.
Nor did she pack a knapsack with emergency supplies, as some officials suggested back then.
In '36, they developed a knapsack with armature-a flexible steel frame that fits on the back of the wearer.
In a struggle, Father McDonnell was wounded as the assailants tried to rifle his pants pockets and stole a knapsack with a Bible and other items.
Not long ago Detective Serpico's beard and hair were bushy as a bear's, and his clothes were thoroughly mod and he carried a revolver jammed in his belt and sometimes a knapsack with other revolvers in it.
MOSCOW — The boy was only 7, but he walked off the plane that arrived in Moscow from Washington all alone, carrying a knapsack with magic markers and candy, along with a single typewritten note.
At a certain point, Feltrinelli lectured the Red Brigades on the necessity of each member's having a "guerrilla knapsack," with a change of clothes, new fake identity cards and "a bag of salt and cigars".
It provided a knapsack stuffed with cash.
Gbessay wore a knapsack filled with case-investigation forms and kept his thumbs hooked on the chest-strap clipped across his sternum.
He recalls how a US serviceman on R&R from Vietnam delivered a knapsack filled with Thai stick to him in Hawaii and how Acapulco gold was burned yellow by the wind coming off the Pacific.
Wearing new shorts, new running shoes and a bad haircut, he said he carried no hard feelings, no chip on his shoulder, only a knapsack filled with a tent and reflective vest, nylon leggings, a sleeping bag, food, water and an A.T.M. card.
